1. Field of the Invention
The present invention relates to a method of measuring the interaction of a fluid with the wall delimiting the fluid, comprising illuminating tracer particles and determining their movement with the fluid by measuring the light scattered. The invention also relates to an apparatus for implementing the method thereof.
2. Description of the Background
Certain additives when added to water or aqueous solutions cause these liquids, at a given pressure gradient, to flow through conduits at a considerably greater speed than in the absence of the additives thereof (e.g., ethylene oxide derivatives, polyacrylamides). Encompassed is also an effect called "drag reduction" which is efficiently utilized in the industry, for example, in fire fighting, to increase the volume of streams in waste water channel networks subjected to sudden loads or for increasing the speed of ships and underwater projectiles. Polyisobutylenes produce drag reduction in oils and are used to reduce the pumping output--to 80%--in pipelines. Such reduction of flow resistance occurs not only at supercritical Re numbers but also in laminar streams. For supercritical Re numbers, the reduction in resistance is explained as an attenuation of turbulence, and thus of turbulent energy production and dissipation in the interface layer at the wall as well as in the free stream. For laminar streams, this is explained on the assumption that the fluid layer in contact with the wall already moves at a finite velocity so that the velocity profile deviates toward higher values compared to the velocity profile of water, thus resulting in slipping. The water profile corresponds to the "theoretical" velocity profile calculated with assumed adhesion (nonslip) conditions for streams which are not otherwise influenced by fluid/wall interactions.
Conversely, under certain flow conditions, the same polymers are also able to increase flow resistance. For example, the addition of about 100 ppm Praestol to streams produces an increase in flow resistance of up to 30 times the value for pure water. These increased pressure losses are explained by assuming an absorption of energy by the macromolecules when they expand in acceleration regions (expansion streams) and not by wall effects. Reference points exist which indicate that an increase in the resistance of other fluids (gelatine solutions) can also be produced by wall effects, namely by a phenomenon opposite to slipping, i.e., a delay of the stream near the walls which results in sticking.
Experimental tests with anionic Praestol which is a macromolecular electrolyte indicate that for low Re number streams this behavior is not only a function of the concentration, and thus of the viscosity of the macromolecules, but also of their charge state. Such behavior, therefore, changes considerably with the concentration of strong electrolytes and with the pH of the solvent.
A question can be posed of whether interactions with the walls resulting in slipping or sticking also occur in blood or blood plasma. In such a case, changes in the pH and the electrolyte composition could have an effect on blood flow. In the larger blood vessels, changes in flow near the walls could influence the mechanical stress on the endothelium but could also have an effect on thrombocyte adhesion. In the capillary system, expanded flow would have an effect primarily on the volume of streams. Whether such mechanisms could play a part in circulatory malfunction and also in the physiological control of capillary circulation remains to be studied.